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Re: ­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of [#permalink]
Expert Reply
 
Bunuel wrote:
KarishmaB wrote:
pintukr wrote:
­If xy = 3(x + 1) + x; and, x and y are integers, x could be any of the following values except:

A. 7
B. 5
C. 4
D. 3
E. 2­




 

You have likely forgotten a 'not' in the question. cannot be any of the following values except... 
Or the question is different in some other way. Please check. 
­

­
I think it soulf be ­"xy = 3(x + 1) + y", instead of "xy = 3(x + 1) + x". In this case x cannot be 5.

 

Ah yes, Bunuel ! The question would become logical then.­
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­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of [#permalink]
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xy - 3x - y = 3

xy - 3x - y + 3 = 6

(x - 1)(y - 3) = 6

(1, 6) (-1, -6) (6, 1) (-6, -1) (2, 3) (3, 2) (-2, -3) (-3, -2)

x - 1 = 1, -1, 6, -6, 2, 3, -2, -3

x = 2, 0, 7, -5, 3, 4, -1, -2­
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Re: ­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of [#permalink]
What's the best way to solve a problem like this?
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Re: ­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of [#permalink]
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Lodz697 wrote:
­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of the following values except:

A. 7
B. 5
C. 4
D. 3
E. 2­

What's the best way to solve a problem like this?

­
You can use the method shown here. Or do the following:

There are several ways to solve this question through algebraic manipulations. You can use the method shown here. Or do the following:

Essentially, our goal is to arrive at an equation of the form \(x = expression \ with \ y \ only\) or \(y = expression \ with \ x \ only\). 

\(xy = 3(x + 1) + y\)

\(xy -y= 3(x + 1)\)

\(y(x-1) = 3x + 3\)

\(y=\frac{3x + 3}{x-1}\)

Next, let's attempt to split the fraction to isolate \(x\) in a single term:

\(y=\frac{3x -3 + 3 + 3}{x-1}\)

\(y=\frac{3x -3 }{x-1}+ \frac{3+3 }{x-1}\)

\(x=3+ \frac{6}{x-1}\)

Since \(y\) is an integer, for \(3+ \frac{6}{x-1}\) to be an integer, \(\frac{6}{x-1}\) must also be an integer. This means that \(x-1\) must be a divisor of 6. From the options, only if x = 5, x - 1 is not a factor of 6, making it the correct answer.

Answer: B.
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Re: ­If xy = 3(x + 1) + y; and, x and y are integers, x could be any of [#permalink]
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